Astarion loses his protection from Cazador, and his only advantage (sunlight resistance). There is a dialogue where he tries to convince Tav about keeping the tadpole and that it is worth the dreams. In another dialogue he says it was the mind flayers that saved him from Cazador, so he doesn't view them the same as Raphael.
So Astarion being pro-tadpole makes sense, same with Gale and Raphael.
Lae'zel on the other hand just doesn't make sense to approve of anything pro-tadpole or pro-Absolute, because that is a betrayal of everyrlthing githyanki believe in. It felt the same when I was playing an evil gith wizard, there was just no reason to side with the crazy drow.